1. 5: DataLink Layer5a-1 Chapter 5 Data Link Layer Computer Networking: A Top Down Approach Featuring the Internet, 2 nd edition. Jim Kurose, Keith Ross Addison-Wesley, July 2002. A note on the use of these ppt slides: We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:  If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)  If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material. Thanks and enjoy! JFK/KWR All material copyright 1996-2002 J.F Kurose and K.W. Ross, All Rights Reserved

2. 5: DataLink Layer5a-2 Chapter 5: The Data Link Layer Our goals: r understand principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing m reliable data transfer, flow control: done! r instantiation and implementation of various link layer technologies

3. 5: DataLink Layer5a-3 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

4. 5: DataLink Layer5a-4 Link Layer: Introduction Some terminology: r hosts and routers are nodes (bridges and switches too) r communication channels that connect adjacent nodes along communication path are links m wired links m wireless links m LANs r 2-PDU is a frame, encapsulates datagram “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

5. 5: DataLink Layer5a-5 Link layer: context r Datagram transferred by different link protocols over different links: m e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link r Each link protocol provides different services m e.g., may or may not provide rdt over link transportation analogy r trip from Princeton to Lausanne m limo: Princeton to JFK m plane: JFK to Geneva m train: Geneva to Lausanne r tourist = datagram r transport segment = communication link r transportation mode = link layer protocol r travel agent = routing algorithm

6. 5: DataLink Layer5a-6 Link Layer Services r Framing, link access: m encapsulate datagram into frame, adding header, trailer m channel access if shared medium m ‘physical addresses’ used in frame headers to identify source, dest different from IP address! r Reliable delivery between adjacent nodes m we learned how to do this already (chapter 3)! m seldom used on low bit error link (fiber, some twisted pair) m wireless links: high error rates Q: why both link-level and end-end reliability?

7. 5: DataLink Layer5a-7 Link Layer Services (more) r Flow Control: m pacing between adjacent sending and receiving nodes r Error Detection: m errors caused by signal attenuation, noise. m receiver detects presence of errors: signals sender for retransmission or drops frame r Error Correction: m receiver identifies and corrects bit error(s) without resorting to retransmission r Half-duplex and full-duplex m with half duplex, nodes at both ends of link can transmit, but not at same time

8. 5: DataLink Layer5a-8 Adaptors Communicating r link layer implemented in “adaptor” (aka NIC) m Ethernet card, PCMCI card, 802.11 card r sending side: m encapsulates datagram in a frame m adds error checking bits, rdt, flow control, etc. r receiving side m looks for errors, rdt, flow control, etc m extracts datagram, passes to rcving node r adapter is semi- autonomous r link & physical layers sending node frame rcving node datagram frame adapter link layer protocol

9. 5: DataLink Layer5a-9 Chapter 5 outline r 5.1 Introduction and services r 5.2 Framing Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

10. 5: DataLink Layer5a-10 Framing Relationship between packets and frames.

11. 5: DataLink Layer5a-11 Framing A character stream. (a) Without errors. (b) With one error.

12. 5: DataLink Layer5a-12 Framing r (a) A frame delimited by flag bytes. r (b) Four examples of byte sequences before and after stuffing.

13. 5: DataLink Layer5a-13 Framing r Bit stuffing (Framing starts and ends with 111111) r (a) The original data. r (b) The data as they appear on the line. r (c) The data as they are stored in receiver’s memory after destuffing.

14. 5: DataLink Layer5a-14 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction

15. 5: DataLink Layer5a-15 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0

16. 5: DataLink Layer5a-16 Internet checksum Sender: r treat segment contents as sequence of 16-bit integers r checksum: addition (1’s complement sum) of segment contents r sender puts checksum value into UDP checksum field Receiver: r compute checksum of received segment r check if computed checksum equals checksum field value: m NO - error detected m YES - no error detected. But maybe errors nonetheless? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)

17. 5: DataLink Layer5a-17 Checksumming: Cyclic Redundancy Check r view data bits, D, as a binary number r choose r+1 bit pattern (generator), G r goal: choose r CRC bits, R, such that m exactly divisible by G (modulo 2) m receiver knows G, divides by G. If non-zero remainder: error detected! m can detect all burst errors less than r+1 bits r widely used in practice (ATM, HDCL)

18. 5: DataLink Layer5a-18 CRC Example Want: D. 2 r XOR R = nG equivalently: D. 2 r = nG XOR R equivalently: if we divide D. 2 r by G, want remainder R R = remainder[ ] D.2rGD.2rG

19. 5: DataLink Layer5a-19 Error Correction r Hamming Code m The bits (r many) that are powers of 2 (1,2,4,8,…)are check bits m The rest (3,5,6,7,9…) are filled up with m data bits m Total number of transmitted bits are m+r r In theory in order to correct one bit error:

20. 5: DataLink Layer5a-20 r Actual data 1001000 r Now m = 7 then r must be 4 _ _ _ _ _ _ _ _ _ _ _ Hamming Code 1 0 0 1 0 00 1 2 3 45 6 7 8 9 10 11 Now calculate parity 3 = 2+1 5 = 4+1 6 = 4+2 7 = 4+2+1 9 = 8+1 10 = 8+2 11 = 8+2+1 0 1 0 0

21. 5: DataLink Layer5a-21 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

22. 5: DataLink Layer5a-22 Multiple Access Links and Protocols Two types of “links”: r point-to-point m PPP for dial-up access m point-to-point link between Ethernet switch and host r broadcast (shared wire or medium) m traditional Ethernet m upstream HFC m 802.11 wireless LAN

23. 5: DataLink Layer5a-23 Multiple Access protocols r single shared broadcast channel r two or more simultaneous transmissions by nodes: interference m only one node can send successfully at a time multiple access protocol r distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit r communication about channel sharing must use channel itself! r what to look for in multiple access protocols:

24. 5: DataLink Layer5a-24 Ideal Mulitple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: m no special node to coordinate transmissions m no synchronization of clocks, slots 4. Simple

25. 5: DataLink Layer5a-25 MAC Protocols: a taxonomy Three broad classes: r Channel Partitioning m divide channel into smaller “pieces” (time slots, frequency, code) m allocate piece to node for exclusive use r Random Access m channel not divided, allow collisions m “recover” from collisions r “Taking turns” m tightly coordinate shared access to avoid collisions

26. 5: DataLink Layer5a-26 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access r access to channel in "rounds" r each station gets fixed length slot (length = pkt trans time) in each round r unused slots go idle r example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle r TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. r FDM (Frequency Division Multiplexing): frequency subdivided.

27. 5: DataLink Layer5a-27 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access r channel spectrum divided into frequency bands r each station assigned fixed frequency band r unused transmission time in frequency bands go idle r example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle r TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. r FDM (Frequency Division Multiplexing): frequency subdivided. frequency bands time

28. 5: DataLink Layer5a-28 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) r unique “code” assigned to each user; i.e., code set partitioning r used mostly in wireless broadcast channels (cellular, satellite, etc) r all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data r encoded signal = (original data) X (chipping sequence) r decoding: inner-product of encoded signal and chipping sequence r allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

29. 5: DataLink Layer5a-29 CDMA Encode/Decode

30. 5: DataLink Layer5a-30 CDMA: two-sender interference

31. 5: DataLink Layer5a-31 Random Access Protocols r When node has packet to send m transmit at full channel data rate R. m no a priori coordination among nodes r two or more transmitting nodes -> “collision”, r random access MAC protocol specifies: m how to detect collisions m how to recover from collisions (e.g., via delayed retransmissions) r Examples of random access MAC protocols: m slotted ALOHA m ALOHA m CSMA, CSMA/CD, CSMA/CA

32. 5: DataLink Layer5a-32 Slotted ALOHA Assumptions r all frames same size r time is divided into equal size slots, time to transmit 1 frame r nodes start to transmit frames only at beginning of slots r nodes are synchronized r if 2 or more nodes transmit in slot, all nodes detect collision Operation r when node obtains fresh frame, it transmits in next slot r no collision, node can send new frame in next slot r if collision, node retransmits frame in each subsequent slot with prob. p until success

33. 5: DataLink Layer5a-33 Slotted ALOHA Pros r single active node can continuously transmit at full rate of channel r highly decentralized: only slots in nodes need to be in sync r simple Cons r collisions, wasting slots r idle slots r nodes may be able to detect collision in less than time to transmit packet

34. 5: DataLink Layer5a-34 Slotted Aloha efficiency r Suppose N nodes with many frames to send, each transmits in slot with probability p r prob that 1st node has success in a slot = p(1-p) N-1 r prob that any node has a success = Np(1-p) N-1 r For max efficiency with N nodes, find p* that maximizes Np(1-p) N-1 r For many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives 1/e =.37 Efficiency is the long-run fraction of successful slots when there’s many nodes, each with many frames to send At best: channel used for useful transmissions 37% of time!

35. 5: DataLink Layer5a-35 Pure (unslotted) ALOHA r unslotted Aloha: simpler, no synchronization r when frame first arrives m transmit immediately r collision probability increases: m frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1]

36. 5: DataLink Layer5a-36 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [p 0 -1,p 0 ]. P(no other node transmits in [p 0 -1,p 0 ] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) … choosing optimum p and then letting n -> infty... = 1/(2e) =.18 Even worse !

37. 5: DataLink Layer5a-37 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: r If channel sensed idle: transmit entire frame r If channel sensed busy, defer transmission r Human analogy: don’t interrupt others!

38. 5: DataLink Layer5a-38 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability

39. 5: DataLink Layer5a-39 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA m collisions detected within short time m colliding transmissions aborted, reducing channel wastage r collision detection: m easy in wired LANs: measure signal strengths, compare transmitted, received signals m difficult in wireless LANs: receiver shut off while transmitting r human analogy: the polite conversationalist

40. 5: DataLink Layer5a-40 CSMA/CD collision detection

41. 5: DataLink Layer5a-41 “Taking Turns” MAC protocols channel partitioning MAC protocols: m share channel efficiently and fairly at high load m inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols m efficient at low load: single node can fully utilize channel m high load: collision overhead “taking turns” protocols look for best of both worlds!

42. 5: DataLink Layer5a-42 “Taking Turns” MAC protocols Polling: r master node “invites” slave nodes to transmit in turn r concerns: m polling overhead m latency m single point of failure (master) Token passing: r control token passed from one node to next sequentially. r token message r concerns: m token overhead m latency m single point of failure (token)

43. 5: DataLink Layer5a-43 Summary of MAC protocols r What do you do with a shared media? m Channel Partitioning, by time, frequency or code Time Division,Code Division, Frequency Division m Random partitioning (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet m Taking Turns polling from a central site, token passing

44. 5: DataLink Layer5a-44 LAN technologies Data link layer so far: m services, error detection/correction, multiple access Next: LAN technologies m addressing m Ethernet m hubs, bridges, switches m 802.11 m PPP m ATM

45. 5: DataLink Layer5a-45 LAN Addresses and ARP 32-bit IP address: r network-layer address r used to get datagram to destination IP network (recall IP network definition) LAN (or MAC or physical or Ethernet) address: r used to get datagram from one interface to another physically-connected interface (same network) r 48 bit MAC address (for most LANs) burned in the adapter ROM

46. 5: DataLink Layer5a-46 LAN Addresses and ARP Each adapter on LAN has unique LAN address

47. 5: DataLink Layer5a-47 LAN Address (more) r MAC address allocation administered by IEEE r manufacturer buys portion of MAC address space (to assure uniqueness) r Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address r MAC flat address => portability m can move LAN card from one LAN to another r IP hierarchical address NOT portable m depends on IP network to which node is attached

48. 5: DataLink Layer5a-48 Recall earlier routing discussion 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E Starting at A, given IP datagram addressed to B: r look up net. address of B, find B on same net. as A r link layer send datagram to B inside link-layer frame B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload datagram frame frame source, dest address datagram source, dest address

49. 5: DataLink Layer5a-49 ARP: Address Resolution Protocol r Each IP node (Host, Router) on LAN has ARP table r ARP Table: IP/MAC address mappings for some LAN nodes m TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B knowing B’s IP address?

50. 5: DataLink Layer5a-50 ARP protocol r A wants to send datagram to B, and A knows B’s IP address. r Suppose B’s MAC address is not in A’s ARP table. r A broadcasts ARP query packet, containing B's IP address m all machines on LAN receive ARP query r B receives ARP packet, replies to A with its (B's) MAC address m frame sent to A’s MAC address (unicast) r A caches (saves) IP-to- MAC address pair in its ARP table until information becomes old (times out) m soft state: information that times out (goes away) unless refreshed r ARP is “plug-and-play”: m nodes create their ARP tables without intervention from net administrator

51. 5: DataLink Layer5a-51 Routing to another LAN walkthrough: send datagram from A to B via R assume A knows B IP address r Two ARP tables in router R, one for each IP network (LAN) r In routing table at source Host, find router 111.111.111.110 r In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc A R B

52. 5: DataLink Layer5a-52 r A creates datagram with source A, destination B r A uses ARP to get R’s MAC address for 111.111.111.110 r A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram r A’s data link layer sends frame r R’s data link layer receives frame r R removes IP datagram from Ethernet frame, sees its destined to B r R uses ARP to get B’s physical layer address r R creates frame containing A-to-B IP datagram sends to B A R B

53. 5: DataLink Layer5a-53 Ethernet “dominant” LAN technology: r cheap $20 for 100Mbs! r first widely used LAN technology r Simpler, cheaper than token LANs and ATM r Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch

54. 5: DataLink Layer5a-54 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: r 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 r used to synchronize receiver, sender clock rates

55. 5: DataLink Layer5a-55 Ethernet Frame Structure (more) r Addresses: 6 bytes m if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol m otherwise, adapter discards frame r Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) r CRC: checked at receiver, if error is detected, the frame is simply dropped

56. 5: DataLink Layer5a-56 Unreliable, connectionless service r Connectionless: No handshaking between sending and receiving adapter. r Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter m stream of datagrams passed to network layer can have gaps m gaps will be filled if app is using TCP m otherwise, app will see the gaps

57. 5: DataLink Layer5a-57 Ethernet uses CSMA/CD r No slots r adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense r transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection r Before attempting a retransmission, adapter waits a random time, that is, random access

58. 5: DataLink Layer5a-58 Ethernet CSMA/CD algorithm 1. Adaptor gets datagram from and creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the mth collision, adapter chooses a K at random from {0,1,2,…,2 m -1}. Adapter waits K*512 bit times and returns to Step 2

59. 5: DataLink Layer5a-59 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Bit time:.1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential Backoff: r Goal: adapt retransmission attempts to estimated current load m heavy load: random wait will be longer r first collision: choose K from {0,1}; delay is K x 512 bit transmission times r after second collision: choose K from {0,1,2,3}… r after ten collisions, choose K from {0,1,2,3,4,…,1023} See/interact with Java applet on AWL Web site: highly recommended !

60. 5: DataLink Layer5a-60 CSMA/CD efficiency r T prop = max prop between 2 nodes in LAN r t trans = time to transmit max-size frame r Efficiency goes to 1 as t prop goes to 0 r Goes to 1 as t trans goes to infinity r Much better than ALOHA, but still decentralized, simple, and cheap

61. 5: DataLink Layer5a-61 Ethernet Technologies: 10Base2 r 10: 10Mbps; 2: under 200 meters max cable length r thin coaxial cable in a bus topology r repeaters used to connect up to multiple segments r repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! r has become a legacy technology

62. 5: DataLink Layer5a-62 10BaseT and 100BaseT r 10/100 Mbps rate; latter called “fast ethernet” r T stands for Twisted Pair r Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub r Hubs are essentially physical-layer repeaters: m bits coming in one link go out all other links m no frame buffering m no CSMA/CD at hub: adapters detect collisions m provides net management functionality hub nodes

63. 5: DataLink Layer5a-63 Manchester encoding r Used in 10BaseT, 10Base2 r Each bit has a transition r Allows clocks in sending and receiving nodes to synchronize to each other m no need for a centralized, global clock among nodes! r Hey, this is physical-layer stuff!

64. 5: DataLink Layer5a-64 Gbit Ethernet r use standard Ethernet frame format r allows for point-to-point links and shared broadcast channels r in shared mode, CSMA/CD is used; short distances between nodes to be efficient r uses hubs, called here “Buffered Distributors” r Full-Duplex at 1 Gbps for point-to-point links r 10 Gbps now !

65. 5: DataLink Layer5a-65 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

66. 5: DataLink Layer5a-66 Interconnecting LAN segments r Hubs r Bridges r Switches m Remark: switches are essentially multi-port bridges. m What we say about bridges also holds for switches!

67. 5: DataLink Layer5a-67 Interconnecting with hubs r Backbone hub interconnects LAN segments r Extends max distance between nodes r But individual segment collision domains become one large collision domain m if a node in CS and a node EE transmit at same time: collision r Can’t interconnect 10BaseT & 100BaseT

68. 5: DataLink Layer5a-68 Bridges r Link layer device m stores and forwards Ethernet frames m examines frame header and selectively forwards frame based on MAC dest address m when frame is to be forwarded on segment, uses CSMA/CD to access segment r transparent m hosts are unaware of presence of bridges r plug-and-play, self-learning m bridges do not need to be configured

69. 5: DataLink Layer5a-69 Bridges: traffic isolation r Bridge installation breaks LAN into LAN segments r bridges filter packets: m same-LAN-segment frames not usually forwarded onto other LAN segments m segments become separate collision domains bridge collision domain collision domain = hub = host LAN (IP network) LAN segment

70. 5: DataLink Layer5a-70 Forwarding How do determine to which LAN segment to forward frame? Looks like a routing problem...

71. 5: DataLink Layer5a-71 Self learning r A bridge has a bridge table r entry in bridge table: m (Node LAN Address, Bridge Interface, Time Stamp) m stale entries in table dropped (TTL can be 60 min) r bridges learn which hosts can be reached through which interfaces m when frame received, bridge “learns” location of sender: incoming LAN segment m records sender/location pair in bridge table

72. 5: DataLink Layer5a-72 Filtering/Forwarding When bridge receives a frame: index bridge table using MAC dest address if entry found for destination then{ if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

73. 5: DataLink Layer5a-73 Bridge example Suppose C sends frame to D and D replies back with frame to C. r Bridge receives frame from from C m notes in bridge table that C is on interface 1 m because D is not in table, bridge sends frame into interfaces 2 and 3 r frame received by D

74. 5: DataLink Layer5a-74 Bridge Learning: example r D generates frame for C, sends r bridge receives frame m notes in bridge table that D is on interface 2 m bridge knows C is on interface 1, so selectively forwards frame to interface 1

75. 5: DataLink Layer5a-75 Interconnection without backbone r Not recommended for two reasons: - single point of failure at Computer Science hub - all traffic between EE and SE must path over CS segment

76. 5: DataLink Layer5a-76 Backbone configuration Recommended !

77. 5: DataLink Layer5a-77 Bridges Spanning Tree r for increased reliability, desirable to have redundant, alternative paths from source to dest r with multiple paths, cycles result - bridges may multiply and forward frame forever r solution: organize bridges in a spanning tree by disabling subset of interfaces Disabled

78. 5: DataLink Layer5a-78 Some bridge features r Isolates collision domains resulting in higher total max throughput r limitless number of nodes and geographical coverage r Can connect different Ethernet types r Transparent (“plug-and-play”): no configuration necessary

79. 5: DataLink Layer5a-79 Bridges vs. Routers r both store-and-forward devices m routers: network layer devices (examine network layer headers) m bridges are link layer devices r routers maintain routing tables, implement routing algorithms r bridges maintain bridge tables, implement filtering, learning and spanning tree algorithms

80. 5: DataLink Layer5a-80 Routers vs. Bridges Bridges + and - + Bridge operation is simpler requiring less packet processing + Bridge tables are self learning - All traffic confined to spanning tree, even when alternative bandwidth is available - Bridges do not offer protection from broadcast storms

81. 5: DataLink Layer5a-81 Routers vs. Bridges Routers + and - + arbitrary topologies can be supported, cycling is limited by TTL counters (and good routing protocols) + provide protection against broadcast storms - require IP address configuration (not plug and play) - require higher packet processing r bridges do well in small (few hundred hosts) while routers used in large networks (thousands of hosts)

82. 5: DataLink Layer5a-82 Ethernet Switches r Essentially a multi- interface bridge r layer 2 (frame) forwarding, filtering using LAN addresses r Switching: A-to-A’ and B- to-B’ simultaneously, no collisions r large number of interfaces r often: individual hosts, star-connected into switch m Ethernet, but no collisions!

83. 5: DataLink Layer5a-83 Ethernet Switches r cut-through switching: frame forwarded from input to output port without awaiting for assembly of entire frame m slight reduction in latency r combinations of shared/dedicated, 10/100/1000 Mbps interfaces

84. 5: DataLink Layer5a-84 Not an atypical LAN (IP network) Dedicated Shared

85. 5: DataLink Layer5a-85 Summary comparison

86. 5: DataLink Layer5a-86 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

87. 5: DataLink Layer5a-87 IEEE 802.11 Wireless LAN r 802.11b m 2.4-5 GHz unlicensed radio spectrum m up to 11 Mbps m direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code m widely deployed, using base stations r 802.11a m 5-6 GHz range m up to 54 Mbps r 802.11g m 2.4-5 GHz range m up to 54 Mbps r All use CSMA/CA for multiple access r All have base-station and ad-hoc network versions

88. 5: DataLink Layer5a-88 Base station approach r Wireless host communicates with a base station m base station = access point (AP) r Basic Service Set (BSS) (a.k.a. “cell”) contains: m wireless hosts m access point (AP): base station r BSSs combined to form distribution system (DS)

89. 5: DataLink Layer5a-89 Ad Hoc Network approach r No AP (i.e., base station) r wireless hosts communicate with each other m to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z r Applications: m “laptop” meeting in conference room, car m interconnection of “personal” devices m battlefield r IETF MANET (Mobile Ad hoc Networks) working group

90. 5: DataLink Layer5a-90 IEEE 802.11: multiple access r Collision if 2 or more nodes transmit at same time r CSMA makes sense: m get all the bandwidth if you’re the only one transmitting m shouldn’t cause a collision if you sense another transmission r Collision detection doesn’t work: hidden terminal problem

91. 5: DataLink Layer5a-91 IEEE 802.11 MAC Protocol: CSMA/CA 802.11 CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -if sense channel busy then binary backoff 802.11 CSMA receiver - if received OK return ACK after SIFS (ACK is needed due to hidden terminal problem)

92. 5: DataLink Layer5a-92 Collision avoidance mechanisms r Problem: m two nodes, hidden from each other, transmit complete frames to base station m wasted bandwidth for long duration ! r Solution: m small reservation packets m nodes track reservation interval with internal “network allocation vector” (NAV)

93. 5: DataLink Layer5a-93 Collision Avoidance: RTS-CTS exchange r sender transmits short RTS (request to send) packet: indicates duration of transmission r receiver replies with short CTS (clear to send) packet m notifying (possibly hidden) nodes r hidden nodes will not transmit for specified duration: NAV

94. 5: DataLink Layer5a-94 Collision Avoidance: RTS-CTS exchange r RTS and CTS short: m collisions less likely, of shorter duration m end result similar to collision detection r IEEE 802.11 allows: m CSMA m CSMA/CA: reservations m polling from AP

95. 5: DataLink Layer5a-95 A word about Bluetooth r Low-power, small radius, wireless networking technology m 10-100 meters r omnidirectional m not line-of-sight infrared r Interconnects gadgets r 2.4-2.5 GHz unlicensed radio band r up to 721 kbps r Interference from wireless LANs, digital cordless phones, microwave ovens: m frequency hopping helps r MAC protocol supports: m error correction m ARQ r Each node has a 12-bit address

96. 5: DataLink Layer5a-96 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

97. 5: DataLink Layer5a-97 Point to Point Data Link Control r one sender, one receiver, one link: easier than broadcast link: m no Media Access Control m no need for explicit MAC addressing m e.g., dialup link, ISDN line r popular point-to-point DLC protocols: m PPP (point-to-point protocol) m HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!

98. 5: DataLink Layer5a-98 PPP Design Requirements [RFC 1557] r packet framing: encapsulation of network-layer datagram in data link frame m carry network layer data of any network layer protocol (not just IP) at same time m ability to demultiplex upwards r bit transparency: must carry any bit pattern in the data field r error detection (no correction) r connection liveness: detect, signal link failure to network layer r network layer address negotiation: endpoint can learn/configure each other’s network address

99. 5: DataLink Layer5a-99 PPP non-requirements r no error correction/recovery r no flow control r out of order delivery OK r no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

100. 5: DataLink Layer5a- 100 PPP Data Frame r Flag: delimiter (framing) r Address: does nothing (only one option) r Control: does nothing; in the future possible multiple control fields r Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

101. 5: DataLink Layer5a- 101 PPP Data Frame r info: upper layer data being carried r check: cyclic redundancy check for error detection

102. 5: DataLink Layer5a- 102 Byte Stuffing r “data transparency” requirement: data field must be allowed to include flag pattern m Q: is received data or flag? r Sender: adds (“stuffs”) extra byte after each data byte r Receiver: m two 01111110 bytes in a row: discard first byte, continue data reception m single 01111110: flag byte

103. 5: DataLink Layer5a- 103 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

104. 5: DataLink Layer5a- 104 PPP Data Control Protocol Before exchanging network- layer data, data link peers must r configure PPP link (max. frame length, authentication) r learn/configure network layer information m for IP: carry IP Control Protocol (IPCP) msgs (protocol field: 8021) to configure/learn IP address

105. 5: DataLink Layer5a- 105 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

106. 5: DataLink Layer5a- 106 Asynchronous Transfer Mode: ATM r 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture r Goal: integrated, end-end transport of carry voice, video, data m meeting timing/QoS requirements of voice, video (versus Internet best-effort model) m “next generation” telephony: technical roots in telephone world m packet-switching (fixed length packets, called “cells”) using virtual circuits

107. 5: DataLink Layer5a- 107 ATM architecture r adaptation layer: only at edge of ATM network m data segmentation/reassembly m roughly analogous to Internet transport layer r ATM layer: “network” layer m cell switching, routing r physical layer

108. 5: DataLink Layer5a- 108 ATM: network or link layer? Vision: end-to-end transport: “ATM from desktop to desktop” m ATM is a network technology Reality: used to connect IP backbone routers m “IP over ATM” m ATM as switched link layer, connecting IP routers

109. 5: DataLink Layer5a- 109 ATM Adaptation Layer (AAL) r ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below r AAL present only in end systems, not in switches r AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells m analogy: TCP segment in many IP packets

110. 5: DataLink Layer5a-110 ATM Adaptation Layer (AAL) [more] Different versions of AAL layers, depending on ATM service class: r AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation r AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video r AAL5: for data (e.g., IP datagrams) AAL PDU ATM cell User data

111. 5: DataLink Layer5a-111 AAL5 - Simple And Efficient AL (SEAL) r AAL5: low overhead AAL used to carry IP datagrams m 4 byte cyclic redundancy check m PAD ensures payload multiple of 48bytes m large AAL5 data unit to be fragmented into 48- byte ATM cells

112. 5: DataLink Layer5a-112 ATM Layer Service: transport cells across ATM network r analogous to IP network layer r very different services than IP network layer Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes no Order no yes Timing no yes no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?

113. 5: DataLink Layer5a-113 ATM Layer: Virtual Circuits r VC transport: cells carried on VC from source to dest m call setup, teardown for each call before data can flow m each packet carries VC identifier (not destination ID) m every switch on source-dest path maintain “state” for each passing connection m link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. r Permanent VCs (PVCs) m long lasting connections m typically: “permanent” route between to IP routers r Switched VCs (SVC): m dynamically set up on per-call basis

114. 5: DataLink Layer5a-114 ATM VCs r Advantages of ATM VC approach: m QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) r Drawbacks of ATM VC approach: m Inefficient support of datagram traffic m one PVC between each source/dest pair) does not scale (N*2 connections needed) m SVC introduces call setup latency, processing overhead for short lived connections

115. 5: DataLink Layer5a-115 ATM Layer: ATM cell r 5-byte ATM cell header r 48-byte payload m Why?: small payload -> short cell-creation delay for digitized voice m halfway between 32 and 64 (compromise!) Cell header Cell format

116. 5: DataLink Layer5a-116 ATM cell header r VCI: virtual channel ID m will change from link to link thru net r PT: Payload type (e.g. RM cell versus data cell) r CLP: Cell Loss Priority bit m CLP = 1 implies low priority cell, can be discarded if congestion r HEC: Header Error Checksum m cyclic redundancy check

117. 5: DataLink Layer5a-117 ATM Physical Layer (more) Two pieces (sublayers) of physical layer: r Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below r Physical Medium Dependent: depends on physical medium being used TCS Functions: m Header checksum generation: 8 bits CRC m Cell delineation m With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send

118. 5: DataLink Layer5a-118 ATM Physical Layer Physical Medium Dependent (PMD) sublayer r SONET/SDH: transmission frame structure (like a container carrying bits); m bit synchronization; m bandwidth partitions (TDM); m several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps r TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps r unstructured: just cells (busy/idle)

119. 5: DataLink Layer5a-119 IP-Over-ATM Classic IP only r 3 “networks” (e.g., LAN segments) r MAC (802.3) and IP addresses IP over ATM r replace “network” (e.g., LAN segment) with ATM network r ATM addresses, IP addresses ATM network Ethernet LANs Ethernet LANs

120. 5: DataLink Layer5a- 120 IP-Over-ATM Issues: r IP datagrams into ATM AAL5 PDUs r from IP addresses to ATM addresses m just like IP addresses to 802.3 MAC addresses! ATM network Ethernet LANs

121. 5: DataLink Layer5a- 121 Datagram Journey in IP-over-ATM Network r at Source Host: m IP layer maps between IP, ATM dest address (using ARP) m passes datagram to AAL5 m AAL5 encapsulates data, segments cells, passes to ATM layer r ATM network: moves cell along VC to destination r at Destination Host: m AAL5 reassembles cells into original datagram m if CRC OK, datagram is passed to IP

122. 5: DataLink Layer5a- 122 Chapter 5 outline r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 LAN addresses and ARP r 5.5 Ethernet r 5.6 Hubs, bridges, and switches r 5.7 Wireless links and LANs r 5.8 PPP r 5.9 ATM r 5.10 Frame Relay

123. 5: DataLink Layer5a- 123 Frame Relay Like ATM: r wide area network technologies r Virtual-circuit oriented r origins in telephony world r can be used to carry IP datagrams m can thus be viewed as link layers by IP protocol

124. 5: DataLink Layer5a- 124 Frame Relay r Designed in late ‘80s, widely deployed in the ‘90s r Frame relay service: m no error control m end-to-end congestion control

125. 5: DataLink Layer5a- 125 Frame Relay (more) r Designed to interconnect corporate customer LANs m typically permanent VC’s: “pipe” carrying aggregate traffic between two routers m switched VC’s: as in ATM r corporate customer leases FR service from public Frame Relay network (e.g., Sprint, ATT)

126. 5: DataLink Layer5a- 126 Frame Relay (more) r Flag bits, 01111110, delimit frame r address: m 10 bit VC ID field m 3 congestion control bits FECN: forward explicit congestion notification (frame experienced congestion on path) BECN: congestion on reverse path DE: discard eligibility address flags data CRC flags

127. 5: DataLink Layer5a- 127 Frame Relay -VC Rate Control r Committed Information Rate (CIR) m defined, “guaranteed” for each VC m negotiated at VC set up time m customer pays based on CIR r DE bit: Discard Eligibility bit m Edge FR switch measures traffic rate for each VC; marks DE bit m DE = 0: high priority, rate compliant frame; deliver at “all costs” m DE = 1: low priority, eligible for congestion discard

128. 5: DataLink Layer5a- 128 Frame Relay - CIR & Frame Marking r Access Rate: rate R of the access link between source router (customer) and edge FR switch (provider); 64Kbps < R < 1,544Kbps r Typically, many VCs (one per destination router) multiplexed on the same access trunk; each VC has own CIR r Edge FR switch measures traffic rate for each VC; it marks (i.e. DE = 1) frames which exceed CIR (these may be later dropped) r Internet’s more recent differentiated service uses similar ideas

129. 5: DataLink Layer5a- 129 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing, ARP r link layer technologies: Ethernet, hubs, bridges, switches,IEEE 802.11 LANs, PPP, ATM, Frame Relay r journey down the protocol stack now OVER! m next stops: multimedia, security, network management